IE popularity pumped up by how it handles images?

Dave, Dadi has a chess forum (Icelandia based) which he is converting from webbbs to mwforum. We are following his conversion on Tarty's forum. He has written javascript which dynamically creates chessboards where people can move the pieces. However, he came across this problem in the conversion which is caused by an IE "bug" .

I thought you might be interested since it involves website use of images.

Well, I'm dumbfounded. "This behavior is by design." What is that supposed to mean? Is that supposed to be an explanation??

It's very hard to imagine why they would purposely do this but given their track record, Mishoo's "popularity theft" theory makes as much sense as anything.

I know IE has always had caching problems with dynamic content. I believe WebBBS had some issues with it. I don't know if this same behaviour would affect PHP-generated images (it doesn't seem to be directly related) but without knowing why they implemented this behaviour, who knows? The problem might be more widespread than we think.